1. Field of the Invention
The present invention relates to a process for producing a semiconductive porcelain composition having a positive temperature coefficient of resistance (PTC), and a heater employing the semiconductive porcelain composition.
2. Description of the Related Art
BaTiO3 semiconductive porcelain composition has been known as a material showing a positive resistance temperature characteristic. When SrTiO3 or PbTiO3 is added to the BaTiO3 semiconductive porcelain composition, a Curie temperature can be shifted, but only PbTiO3 is an additive material that enables shifting in a positive direction. However, since PbTiO3 contains an element causing environmental pollution, a material using no PbTiO3 as an additive material has been desired. Consequently, a semiconductive porcelain composition in which a part of Ba in BaTiO3 is substituted with Bi—Na was proposed (see Patent Document 1).
Where a BaTiO3-based material is treated by, for example, sintering in a reducing atmosphere so as to decrease room temperature resistivity, there is a problem that a temperature coefficient of resistance (jump characteristic) is decreased. When the jump characteristic is decreased, there is a problem that switching does not occur at the objective temperature. Therefore, to improve the jump characteristic, it is proposed to conduct a heat treatment at high temperature exceeding 1,100° C. (Patent Document 2).    Patent Document 1: WO 2006/106910    Patent Document 2: JP-A-56-169301